Substantial attention has been directed in recent years toward composite materials capable of exhibiting negative effective permeability and/or negative effective permittivity with respect to incident electromagnetic radiation. Such materials, often interchangeably termed artificial materials or metamaterials, generally comprise periodic arrays of electromagnetically resonant cells that are of substantially small dimension (e.g., one-fifth or less) compared to the wavelength of the incident radiation. Although the individual response of any particular cell to an incident wavefront can be quite complicated, the aggregate response the resonant cells can be described macroscopically, as if the composite material were a continuous material, except that the permeability term is replaced by an effective permeability and the permittivity term is replaced by an effective permittivity. However, unlike continuous materials, the resonant cells have structures that can be manipulated to vary their magnetic and electrical properties, such that different ranges of effective permeability and/or effective permittivity can be achieved across various useful radiation wavelengths.
Of particular appeal are so-called negative index materials, often interchangeably termed left-handed materials or negatively refractive materials, in which the effective permeability and effective permittivity are simultaneously negative for one or more wavelengths depending on the size, structure, and arrangement of the resonant cells. Potential industrial applicabilities for negative-index materials include so-called superlenses having the ability to image far below the diffraction limit to λ/6 and beyond, new designs for airborne radar, high resolution nuclear magnetic resonance (NMR) systems for medical imaging, microwave lenses, and other radiation processing devices.
One issue that arises in the realization of useful devices from such composite materials, including negative index materials, relates to isotropy of response and amenability to large scale fabrication processes. For example, dense planar arrays of two-dimensional resonant cells having electrical conductors parallel to a substrate are generally amenable to large scale lithographic fabrication processes. However, their response can be anisotropic because, for example, resonance for the magnetic field is favored for magnetic field vectors normal to the plane of the substrate and resonance for the electric field is favored for electrical field vectors parallel to the plane of the substrate. On the other hand, composite materials having three-dimensional resonant cells in which there are electrical conductors for each of three orthogonal planes can provide increased isotropy of response, but are substantially more difficult to fabricate on a large scale than composite materials having planar arrays of two-dimensional resonant cells.
Another issue that arises relates to wavelengths of operation and isotropy of response, with three-dimensional resonant cells being difficult to fabricate for smaller wavelengths such as those in the infrared and optical regimes. It would be desirable to provide a composite material that is amenable to large scale fabrication processes while also having increased isotropy of response. It would be further desirable to provide such composite material that can be operable for smaller wavelengths such as those in the infrared and optical regimes. Other issues arise as would be apparent to one skilled in the art in view of the present disclosure.